This invention relates to digital-to-analog converters. More particularly, the invention relates to digital-to-analog converters in which a plurality of binary indications representing a value are converted to an analog current or an analog voltage representing the value without any ringing during the binary indications or at the transitions between successive binary indications.
Most parameters such as measurements of temperature, humidity and pressure are analog. For example, the use of a mercury thermometer to measure the temperature of a patient is analog since the temperature is measured by the rise of a mercury column. However, temperature may also be indicated digitally. For example, an indication of a temperature of xe2x80x9c98.6xe2x80x9d may be provided digitally by providing three separate indications of xe2x80x9c9xe2x80x9d, xe2x80x9c8xe2x80x9d and xe2x80x9c6xe2x80x9d.
Generally, when parameters such as temperature or pressure are measured on an analog basis and these measurements are used to provide calculations for controlling the operation of a system in which the values of temperature and pressure are regulated, the analog values are converted to digital values for providing the calculations. The calculations are then converted to digital values to provide the regulation of the parameters such as temperature and pressure.
Integrated circuit chips are generally provided for converting digital indications of a value to an analog representation of the value. Preferably this conversion is provided in as short a time (or as high a frequency) as possible. Minimizing the time for the conversion is desirable because it provides for an enhanced regulation of the values of parameters such as pressure and temperature.
Integrated circuit chips have been progressively provided through the years with decreased micron size. In other words, the thickness of the electrical leads connecting the different components in the electrical circuitry on the integrated circuit chip has been progressively decreased through the years. For example, the micron size of the electrical leads on an integrated circuit chip have progressively decreased in size during the past ten (10) years from approximately two (2) microns to approximately one half micron (0.5xcexc) or less. Decreases in micron size have produced corresponding increases in the frequency at which the electrical circuits on the integrated circuit chip are able to operate. For example, electrical circuits made from CMOS technology on an integrated circuit chip are now able to operate at frequencies in the order of several hundred megahertz in comparison to frequencies less than one hundred megahertz (100 Mhz) ten years ago.
Digital-to-analog converters have problems of ringing, particularly when they operate at high frequencies. The ringing occurs during the period of each of the binary indications. The ringing also occurs at the transitions between successive ones of the binary indications. The ringing obscures the generation of the analog current or analog voltage which represents the cumulative value of the binary indications. The ringing becomes pronounced because of the high frequencies at which the digital-to-analog converters operate. As previously indicated, these high frequencies are provided because of the progressive decrease in the micron size of the electrical leads, and the progressive decrease in the dimensions of devices such as transistors, in the integrated circuit chips.
In one embodiment of the invention, binary indications are converted to an analog representation with significant reductions in ringing at the transitions between successive binary indications or in the period during each binary indication. The binary indications are disposed in a row-and-column matrix to provide a thermometer code. The converter includes pluralities of decoders and latches, each decoder being associated with an individual latch. Each decoder responds to binary indications of an individual row and an individual column and the next column to produce a latched pair of output indications, inverted relative to each other, in synchronism with a clock signal.
The production of the latched outputs in synchronism with the clock signal inhibits ringing in the period during each binary indication. Each pair of inverted latch outputs is respectively introduced to a differential amplifier, formed from MOS transistors of the p type, in an individual one of a plurality of current sources. Each differential amplifier has a pair of branches each responsive to the paired inverted outputs from the associated latch in an opposite relationship to that of the other branch.
The p type transistors in each differential amplifier inhibit ringing in such amplifier at the transitions. between the successive binary indications. Each branch in each differential amplifier is connected to a resistor common to the corresponding branches in the other differential amplifiers. Such branches pass through such resistor a current dependent upon the cumulative current through such branches. This cumulative current provides the analog representation.